1. Field of the Invention
The invention is based on a method for determining the load capacity of an AC network as claimed in the preamble of patent claim 1.
2. Discussion of Background
In the preamble to patent claim 1, the invention refers to a prior art as is known from DE 4037531 A1. A method is described there for controlling rectifiers of converters for three-phase traction vehicles, in which the four-quadrant controllers in the converters are clocked with phase shifts. This reduces the overall effect on the feeding AC network of harmonics which occur when clocking a plurality of rectifiers.
In low-power electrical networks, the operation of electric railway vehicles is frequently made more difficult owing to the fact that stability problems occur in the overall network as a result of an interaction between vehicles, network impedances and substations. In such electrical networks, an increased power demand from a vehicle immediately results in an amplitude and phase change in the voltage for the other vehicles. Such stability and power matching problems at the fundamental frequency of, for example, 162/3 Hz or 50 Hz frequently occur in countries where the population density is low and there is little industry.
DE 195 04 271 C1 proposes that the impedance of an AC network be measured by feeding a reactive current into the network and measuring the induced phase shift.
DE 41 29 261 C1 discloses a method for digital current regulation for a four-quadrant regulator in an AC network. The current regulating variable used in this case is a future current value which is predicted taking into account a phase angle of the network voltage.
The subject matter of DE-AS 1 088 152 is a device and a method for determining a network internal impedance. A network load impedance is periodically connected, and the complex network internal impedance is determined from the drop in the network voltage.
The relevant prior art furthermore refers to the book by U. Tietze and Ch. Schenk, Halbleiter-Schaltungstechnik [Semiconductor circuit technology], 10th Edition, Springer-Verlag 1993, pages 954-956, from which it is known for phase changes in an oscillating signal to be determined by coasting synchronization. The present invention makes use of this capability with regard to the detected network voltage.